Author name code: bard ADS astronomy entries on 2022-09-14 author:"Bard, S.," ------------------------------------------------------------------------ Title: Radiative Hydrodynamic Simulations of Acoustic Waves in Sunspots Authors: Bard, S.; Carlsson, M. Bibcode: 2010ApJ...722..888B Altcode: We investigate the formation and evolution of the Ca II H line in a sunspot. The aim of our study is to establish the mechanisms underlying the formation of the frequently observed brightenings of small regions of sunspot umbrae known as "umbral flashes." We perform fully consistent NLTE radiation hydrodynamic simulations of the propagation of acoustic waves in sunspot umbrae and conclude that umbral flashes result from increased emission of the local solar material during the passage of acoustic waves originating in the photosphere and steepening to shock in the chromosphere. To quantify the significance of possible physical mechanisms that contribute to the formation of umbral flashes, we perform a set of simulations on a grid formed by different wave power spectra, different inbound coronal radiation, and different parameterized chromospheric heating. Our simulations show that the waves with frequencies in the range 4.5-7.0 mHz are critical to the formation of the observed blueshifts of umbral flashes while waves with frequencies below 4.5 mHz do not play a role despite their dominance in the photosphere. The observed emission in the Ca II H core between flashes only occurs in the simulations that include significant inbound coronal radiation and/or extra non-radiative chromospheric heating in addition to shock dissipation. Title: Constructing Computationally Tractable Models of Si I for the 1082.7 nm Transition Authors: Bard, S.; Carlsson, M. Bibcode: 2008ApJ...682.1376B Altcode: The Si I 1082.7 nm line is often observed together with the He I 1083.0 nm. The silicon line is assumed to give a measure of the photospheric driver of the chromospheric dynamics observed in the He I 1083.0 nm line. To exploit the diagnostic potential of the silicon line it is of interest to study its line formation. We here analyze physical processes in the formation of the Si I 1082.7 nm line and find that it is formed in non-LTE (NLTE). Its line core intensity is lower than the corresponding LTE value because the source function is below the Planck function due to photon losses in the line itself. In order to make possible full NLTE dynamic calculations of this line we develop a general method for reducing the size of atomic models used in NLTE calculations by representing several physically similar energy levels as one mean level. Our procedure preserves all the important physical properties of the atom that have a bearing on the observable spectrographic properties of the line under study. We apply our procedure to the Si I atom for the 1082.7 nm transition under solar conditions, and we are able to reduce the size of the atomic model from 238 levels and 3152 transitions to 23 levels and 171 transitions with no significant change in the calculated emergent intensity of the 1082.7 nm line. We calculate the response functions for the Si I 1082.7 nm line both in the quiet Sun and above sunspot umbrae. We find that the line-center Doppler shift has a mean response height to velocity perturbations of 541 km in a quiet-Sun atmosphere and 308 km in a sunspot atmosphere. These numbers depend on the model atmosphere employed. Title: Radiative hydrodynamic simulations of acoustic waves in sunspots Authors: Bard, S.; Carlsson, M. Bibcode: 2004IAUS..223..445B Altcode: 2005IAUS..223..445B We describe fully consistent NLTE radiation hydrodynamic simulations of the propagation of acoustic waves in sunspot umbrae. In contrast to the case of the quiet internetwork Sun, we find that the observed behavior of the resonance lines of singly ionized calcium (H and K lines) cannot be explained without adding non-acoustic heating. The best agreement with observations is obtained with an extra heating term as a function of height that is constant per volume. Title: Normal modes and discovery of high-order cross-frequencies in the DBV white dwarf GD 358 Authors: Vuille, F.; O'Donoghue, D.; Buckley, D. A. H.; Massacand, C. -M.; Solheim, J. E.; Bard, S.; Vauclair, G.; Giovannini, O.; Kepler, S. O.; Kanaan, A.; Provencal, J. L.; Wood, M. A.; Clemens, J. C.; Kleinman, S. J.; O'Brien, M. S.; Nather, R. E.; Winget, D. E.; Nitta, A.; Klumpe, E. W.; Montgomery, M. H.; Watson, T. K.; Bradley, P. A.; Sullivan, D. J.; Wu, K.; Marar, T. M. K.; Seetha, S.; Ashoka, B. N.; Mahra, H. S.; Bhat, B. C.; Babu, V. C.; Leibowitz, E. M.; Hemar, S.; Ibbetson, P.; Mashals, E.; Meištas, E.; Moskalik, P.; Zoła, S.; Krzesiński, J.; Pajdosz, G. Bibcode: 2000MNRAS.314..689V Altcode: We present a detailed mode identification performed on the 1994 Whole Earth Telescope (WET) run on GD 358. The results are compared with that obtained for the same star from the 1990 WET data. The two temporal spectra show very few qualitative differences, although amplitude changes are seen in most modes, including the disappearance of the mode identified as k=14 in the 1990 data. The excellent coverage and signal-to-noise ratio obtained during the 1994 run lead to the secure identification of combination frequencies up to fourth order, i.e. peaks that are sums or differences of up to four parent frequencies, including a virtually complete set of second-order frequencies, as expected from harmonic distortion. We show how the third-order frequencies are expected to affect the triplet structure of the normal modes by back-interacting with them. Finally, a search for l=2 modes was unsuccessful, not verifying the suspicion that such modes had been uncovered in the 1990 data set. Title: New Whole Earth Telescope observations of CD-24 7599: steps towards delta Scuti star seismology Authors: Handler, G.; Pikall, H.; O'Donoghue, D.; Buckley, D. A. H.; Vauclair, G.; Chevreton, M.; Giovannini, O.; Kepler, S. O.; Goode, P. R.; Provencal, J. L.; Wood, M. A.; Clemens, J. C.; O'Brien, M. S.; Nather, R. E.; Winget, D. E.; Kleinman, S. J.; Kanaan, A.; Watson, T. K.; Nitta, A.; Montgomery, M. H.; Klumpe, E. W.; Bradley, P. A.; Sullivan, D. J.; Wu, K.; Marar, T. M. K.; Seetha, S.; Ashoka, B. N.; Mahra, H. S.; Bhat, B. C.; Babu, V. C.; Leibowitz, E. M.; Hemar, S.; Ibbetson, P. A.; Mashal, E.; Meistas, E. G.; Dziembowski, W. A.; Pamyatnykh, A. A.; Moskalik, P.; Zola, S.; Pajdosz, G.; Krzesinski, J.; Solheim, J. E.; Bard, S.; Massacand, C. M.; Breger, M.; Gelbmann, M. J.; Paunzen, E.; North, P. Bibcode: 1997MNRAS.286..303H Altcode: 92h of new Whole Earth Telescope observations have been acquired for the delta Scuti star CD-24 7599. All the seven pulsation modes reported by Handler et al. are confirmed. However, significant amplitude variations which are not caused by beating of closely spaced frequencies occurred within two years. Analysing the combined data of both WET runs, we detect six further pulsation modes, bringing the total number up to 13. We also examine our data for high-frequency pulsations similar to those exhibited by rapidly oscillating Ap stars, but we do not find convincing evidence for variability in this frequency domain. From new colour photometry and spectroscopy we infer that CD-24 7599 is a hot main-sequence delta Scuti star with approximately solar metallicity and vsini=52& plusmn2kms^-1. We cannot yet propose a definite pulsation mode identification, but we report the detection of a characteristic frequency spacing between the different modes. We ascribe it to the simultaneous presence of l=1 and l=2 modes of consecutive radial order. A comparison of this frequency spacing with frequencies of solar-metallicity models, as well as stability analysis, allows us to constrain tightly the evolutionary state of CD-24 7599. It is in the first half of its main-sequence evolution, and has a mass of 1.85 +/-0.05 M_solar and a mean density of rho^-=0.246+/-0.020rho^-_solar. This yields a seismological distance of 650 +/- 70 pc, which is as accurate as distance determinations for delta Scuti stars observed in clusters. Most of the pulsation modes are pure p modes of radial order k=4-6, but the g_1 mode of l=2 is likely to be excited and observed as well. Since a significant contribution to this mode's kinetic energy comes from the outer part of the convective core, CD-24 7599 becomes particularly interesting for testing convective overshooting theories. Title: Shock Signature in Sunspots Authors: Bard, S.; Carlsson, M. Bibcode: 1997ESASP.404..189B Altcode: 1997cswn.conf..189B No abstract at ADS Title: Model spectra for the helium cataclysmic variable: AM CVn Authors: Bard, S. Bibcode: 1996ASSL..208..133B Altcode: 1996IAUCo.158..133B; 1996cvro.coll..133B No abstract at ADS